Image resize and preserve aspect ratio : #609

[NeerajKomuravalli]

@TobyRoseman As discussed, I have added two basic functions:

1. resize_with_original_aspect_ratio : to resize images by preserving aspect ratio using the user given parameter => min_side_length/max_side_length.
2. resize_annotations : to resize annotations based on any new image dimension provided by the user.

I couldn't find any test cases written for any functions in image_analysis.py in the test folder so I did not add any test cases for these as well.

Let me know if any changes are required to the code submitted.

[TobyRoseman]

linking to open issue: #609.

[NeerajKomuravalli]

Will add test cases In test_image_type.py and push all the modifications you recommended in the next push.